The present invention is directed to a process for forming two-color images. More specifically, the present invention is directed to a process for forming two-color images which comprises charging an imaging member, creating on the member a latent image comprising areas of high, medium, and low potential, developing the low areas of potential with a developer comprises a specific colored toner and a specific carrier as described herein, subsequently developing the high areas of potential with a developer comprising a specific black toner and a specific carrier as described herein, transferring the developed two-color image to a substrate, and optionally permanently affixing the image to the substrate. One advantage of the process of the present invention is the ability to generate high quality two-color images in a single development pass, particularly as a result of the absence of interaction between the black and color developers.
Processes for forming two-colored images are known. For example, U.S. Pat. No. 4,264,185, the disclosure of which is totally incorporated herein by reference, discloses an apparatus for forming two color images by forming a bipolar electrostatic image of a two color original document on a photoconductive drum. A first developing unit applies a toner of a first color and polarity to the drum and a second developing unit applies a toner of a second color and polarity to the drum to form a two color electrostatic image which is transferred and fixed to a copy sheet. A bias voltage of the first polarity is applied to the second developing unit to repel the toner of the first color and polarity against the drum and prevent degradation of the first color toner image. A bias voltage of the second polarity is applied to the first developing unit to prevent contamination of the first color toner with the second color toner.
In addition, U.S. Pat. No. 4,308,821 discloses a method and apparatus for forming two-color images which employs two magnetic brushes. The first developed image is not disturbed during development of the second image, since the second magnetic brush contacts the surface of the imaging member more lightly than the first magnetic brush, and the toner scraping force of the second magnetic brush is reduced in comparison with that of the first magnetic brush by setting the magnetic flux density on a second non-magnetic sleeve with an internally disposed magnet smaller than the magnetic flux density on a first magnetic sleeve, or by adjusting the distance between the second non-magnetic sleeve and the surface of the imaging member. In addition, the toners may have different quantities of electric charge.
Further, U.S. Pat. No. 4,378,415 discloses a method of highlight color imaging which comprises providing a layered organic photoreceptor having a red sensitive layer and a short wavelength sensitive layer, subjecting the imaging member to negative charges, followed by subjecting the imaging member to positive charges, imagewise exposing the member, and developing with a colored developer composition comprising positively charged toner components, negatively charged toner components, and carrier particles. Another U.S. Pat. No. 4,430,402, discloses a two-component type dry developer for use in dichromatic electrophotography which comprises two kinds of developers, each of which consists of a toner and a carrier. Dichromatic images are formed by developing a both positively and negatively electrified electrostatic latent image successively with toners different in polarity and color from each other, wherein one carrier becomes positively charged by friction with either of the two toners while the other carrier becomes negatively charged by friction with either of the two toners.
Additionally, U.S. Pat. No. 4,594,302 discloses a developing process for two-colored electrophotography which comprises charging the surface of a photoreceptor with two photosensitive layers of different spectral sensitivities with one polarity, subsequently charging the photoreceptor with a different polarity, exposing a two-colored original to form electrostatic latent images having different polarities corresponding to the two-colored original, developing one latent image with a first color toner of one polarity, exposing the photoreceptor to eliminate electric charges with the same polarity as the first color toner which are induced on the surface of the photoreceptor in the vicinity of the latent image developed by the first color toner, and developing the other latent image with a second color toner charged with a polarity different from that of the first color toner.
In addition, U.S. Pat. No. 4,500,616 discloses a method of developing electrostatic latent images by selectively extracting colored grains of one polarity from a mixture containing colored grains having opposite polarity to each other in the presence of an alternating field, followed by development of the electrostatic image by the selectively extracted colored grains. Further, U.S. Pat. No. 4,524,117 discloses an electrophotographic method for forming two-colored images which comprises uniformly charging the surface of a photoreceptor having a conductive surface and a photoconductive layer sensitive to a first color formed on the conductive substance, followed by exposing a two-colored original to form on the photoconductive layer a latent image corresponding to a second color region in the original with the same polarity as the electric charges on the surface of the photoconductive layer. The photoreceptor surface is then subjected to a reversal development treatment by the use of a photoconductive color toner charged with the same polarity as the electric charges constituting the latent image, to develop the non-charged region with the photoconductive toner. The latent image is then subjected to normal development treatment with an insulative toner having a color different from the color of the photoconductive toner. Subsequently, the color toners on the photoconductive layer are charged with a different polarity from the charging polarity and, simultaneously, the original is exposed through a filter shielding against the first color, thereby forming a two-colored image.
Another U.S. Pat. No. 4,525,447, the disclosure of which is totally incorporated herein by reference, discloses an image forming method which comprises forming on a photosensitive member an electrostatic latent image having at least three different levels of potentials, or comprising first and second latent images and developing the first and second latent images with a three component developer. The developer comprises a magnetic carrier, a first toner chargeable to one polarity by contact with the magnetic carrier, and a second toner chargeable to a polarity opposite to that of the first toner by contact with the first toner, but substantially not chargeable by contact with the magnetic carrier. Further, U.S. Pat. No. 4,539,281 discloses a method of forming dichromatic copy images by forming an electrostatic latent image having a first image portion and a second image portion. The first image portion is developed by a first magnetic brush with a magnetic toner of a first color that is chargeable to a specific polarity, and the second image portion is developed by a second magnetic brush with a mixture of a magnetic carrier substantially not chargeable with the magnetic toner and a non-magnetic toner of a second color chargeable to a polarity opposite to that of the magnetic toner by contact with the magnetic carrier.
Additionally, U.S. Pat. No. 4,562,129, the disclosure of which is totally incorporated herein by reference, discloses a method of forming dichromatic copy images with a developer composed of a high-resistivity magnetic carrier and a nonmagnetic insulating toner which are triboelectrically chargeable. An electrostatic latent image having at least three different levels of potential is formed and the toner and carrier are adhered respectively onto the first and second image portions. In addition, U.S. Pat. No. 4,640,883, the disclosure of which is totally incorporated herein by reference, discloses a method of forming composite or dichromatic images which comprises forming on an imaging member electrostatic latent images having at least three different potential levels, the first and second latent images being represented respectively by a first potential and a second potential relative to a common background potential. The first and second images are developed by a first magnetic brush using two kinds of toners, at least one of which is magnetic, and both of which are chargeable to polarities opposite to each other, with application to a developing electrode of a bias voltage capable of depositing the magnetic toner on the backdround potential area, to deposit selectively the two toners on the first and second latent images and to deposit the magnetic toner on the background potential area, while collecting the deposited magnetic toner at least from the background potential area by second magnetic brush developing means.
Other representative patents of interest with respect to formation of two-color images include U.S. Pat. Nos. 4,045,218 and 4,572,651.
The process of charging a photoresponsive imaging member to a single polarity and creating on it an image consisting of at least three different levels of potential of the same polarity is disclosed in U.S. Pat. No. 4,078,929, the disclosure of which is totally incorporated herein by reference. This patent discloses a method of creating two colored images by creating on an imaging surface a charge pattern including an area of first charge as a background area, a second area of greater voltage than the first area, and a third area of lesser voltage than the first area, with the second and third areas functioning as image areas. The charge pattern is developed in a first step with positively charged toner particles of a first color, and, in a subsequent development step, developed with negatively charged toner particles of a second color. Alternatively, charge patterns may be developed with a dry developer containing toners of two different colors in a single development step. According to the teachings of this patent, however, the images produced are of inferior quality compared to those developed in two successive development steps. Also of interest with respect to the tri-level process for generating images is U.S. Pat. No. 4,686,163.
Although the known processes for forming two-color images are suitable for their intended purposes, a need continues to exist for processes for forming two-color images. In addition, a need remains for two-color image formation processes wherein the two developers exhibit a low level of mutual interaction. Mutual interaction may occur, for example, when the carrier of the developer used second triboelectrically charges against the toner used first; thus, the first image is developed with the first developer and the second image is subsequently developed with the second developer of different color, and during development of the second image, the carrier of the second developer triboelectrically interacts with the image previously developed with the first toner. This interaction can result in removal of some of the first toner from the substrate, causing reduced image density, and can also lead to contamination of the second developer housing with the first toner. A need also exists for a process for forming two-color images wherein the second developer does not react with, or is triboactively neutral to, the first developed image on the photoreceptor. A need also exists for a process for forming two-color images wherein the first developer does not discharge the latent image to be developed by the second developer. Additionally, there is a need for a two-color image formation process wherein the developers are of specified triboelectric charge, charge distribution, and conductivity, and exhibit acceptable admix times and developer lifetimes. Further, there is a need for a two-color image formation process wherein the two developers exhibit similar rheological properties, thereby enhancing fusing, and similar cleaning latitudes, thereby enhancing cleaning of the photoreceptor.